While I was fighting with the other windows, Eric took on the challenge to replace the first curved window. As shown in the photo the window sash is a standard straight Adlake sash. Added to it is a filler strip which rounds out the gap between the sash and the outer skin. It is attached to the straight sash with small screws. The gap between these two pieces is sealed with rope caulk. We found that Homax #18 Window and Door Caulk worked best for us because of its thin cross section.